Schafer U.S. Pat. Nos. 3,193,437 and 3,193,441, as well as Chant U.S. Pat. No. 3,867,221, illustrate the concept of an initially open-cell foam material impregnated with a thermosetting resin used alone or in combination with reinforcing fibers, and wherein the foam layer is compressed under relatively low pressure such that the thermosetting resin substantially fills the cells of the foam and which resin is cured while maintaining the resilient foam layer in a compressed condition.
Also of interest is applicant's copending application U.S. Ser. No. 902,590, filed May 4, 1978, now U.S. Pat. No. 4,157,416 wherein a composite structure is formed utilizing different resin compositions which, while contemporaneously cured, remain in that discrete layer of the open-cell foam material to which they are introduced and do not flow into an adjacent resin impregnated foam layer during the compression and curing process.
To the best of applicant's knowledge and experience, it has been the preferred practice to incorporate an internal mold-release material in those resin systems where a composite structure is formed under heat and pressure.